Microcellular foam is typically defined as having cell sizes of less than 100 microns and a cell density of greater than 10.sup.6 cells/cm.sup.3 of the original solid material. Generally, the requirements for forming microcellular foams include creating a single-phase solution of polymeric material and physical blowing agent, and subjecting the solution to a thermodynamic instability to create sites of nucleation of very high density which grow into cells.
Methods for molding microcellular material have been described. U.S. Pat. No. 4,473,665 (Martini-Vvedensky) describes a molding system and method for producing microcellular parts. Polymeric pellets are pre-pressurized with a gaseous blowing agent and melted in a conventional extruder to form a solution of blowing agent and molten polymer, which then is extruded into a pressurized mold cavity. The pressure in the mold is maintained above the solubility pressure of the gaseous blowing agent at melt temperatures for the given initial saturation. When the molded part temperature drops to the appropriate critical nucleation temperature, the pressure on the mold is dropped, typically to ambient, and the part is allowed to foam.
U.S. Pat. No. 5,158,986 (Cha et al.) describes an alternative molding system and method for producing microcellular parts. Polymeric pellets are introduced into a conventional extruder and melted. A blowing agent of carbon dioxide in its supercritical state is established in the extrusion barrel and mixed to form a homogenous solution of blowing agent and polymeric material. A portion of the extrusion barrel is heated so that as the mixture flows through the barrel, a thermodynamic instability is created, thereby creating sites of nucleation in the molten polymeric material. The nucleated material is extruded into a pressurized mold cavity. Pressure within the mold is maintained by counter pressure of air. Cell growth occurs inside the mold cavity when the mold cavity is expanded and the pressure therein is reduced rapidly; expansion of the mold provides a molded and foamed article having small cell sizes and high cell densities. Nucleation and cell growth occur separately according to the technique; thermally-induced nucleation takes place in the barrel of the extruder, and cell growth takes place in the mold.
The use of check valves, including ring check valves, is known in injection molding to prevent the molten plastic accumulated at the distal end of a reciprocating screw from flowing backwards during an injection of the plastic into a mold.
The following U.S. Patent Applications describe typical check valve configurations used in plastic processing systems. U.S. Pat. No. 4,512,733 (Eichlseder et al.) describes a check valve on the end of a plastifying screw for an injection molding apparatus. The check valve comprises a valve housing and an axial displacable valve member that is received in this housing.
U.S. Pat. No. 5,164,207 (Durina) describes a plastic extruder having a rotating screw within a cylindrical shell which is used to feed molten plastic to a high pressure injection molding apparatus. An automatic shut off valve is mounted at the forward end of the screw. During the extrusion step, the valve is forced open to allow molten plastic to flow from the extruder to the injection molder. The valve automatically closes under the action of a spring during the high pressure injection molding operation to prevent backflow of plastic through the extruder.
U.S. Pat. No. 5,258,158 (Dray) describes a positive type non-return valve that is used to positively stop the reverse flow of material in injection molding machines. The valve can be connected at a downstream end of the screw with a thread, or can also be an integral part of the screw. The valve allows material to pass when the screw is rotating, but closes when the screw translates forward, as in an injection molding cycle, with no screw rotation.
While the above and other reports represent several techniques and systems associated with the manufacture of foam material and microcellular material, a need exists in the art for improved systems for foam processing, and in particular for microcellular foam processing.
It is, therefore, an object of the invention to provide systems, methods, and articles useful in the production of microcellular foams, and also useful in the production of conventional foams.